Species of Psittacosaurus
Eighteen species have been assigned to the extinct ceratopsian dinosaur genus Psittacosaurus, although only nine to eleven are considered valid.12 This is the highest number of valid species assigned to any single dinosaur genus (not including birds).By contrast, most dinosaur genera contain only a single species. As some species are known only from skull material, species of Psittacosaurus are primarily distinguished by features of the skull and teeth. Several species can be recognized by features of the pelvis as well. Overall size estimates of most species have not been published or are unavailable due to lack of fossil preservation. However, measurements of the skull or femur have been published for all well-established species and can be used as a basis for comparison. Psittacosaurus mongoliensis (type) Psittacosaurus mongoliensis is the type species of the genus, named by American paleontologist Henry Fairfield Osborn in 1923. Remains of this dinosaur were first discovered the year before, on the third American Museum of Natural History expedition to the Gobi Desert of Mongolia, when one of the expedition's drivers found the type specimen (AMNH 6254). This same expedition turned up the remains of many other famous Mongolian dinosaurs, including Protoceratops, Oviraptor, and Velociraptor. Many later expeditions by various combinations of Mongolian, Russian, Chinese, American, Polish, Japanese, and Canadian paleontologists also recovered specimens from throughout Mongolia and northern China. In these areas, Psittacosaurus mongoliensis fossils are found in most sedimentary strata dating to the Aptian to Albian stages of the Early Cretaceous Period, or approximately 125 to 100 Ma (million years ago). Fossil remains of over 75 individuals have been recovered, including nearly 20 complete skeletons with skulls. Individuals of all ages are known, from hatchlings less than 13 centimeters (5 in) long, to very old adults reaching nearly 2 meters (6.5 ft) in length. Skulls of P. mongoliensis are flat on top, especially over the back of the skull, with a triangular depression, the antorbital fossa, on the outside surface of the maxilla (an upper jaw bone). A flange is present on the lower edge of the dentary (the tooth-bearing bone of the lower jaw), although it is not as prominent as in P. meileyingensis or P. major. P. mongoliensis is among the largest known species. The skull of the type specimen, which is probably a juvenile, is 15.2 centimetersm (6 in) long, and the associated femur is 16.2 centimeters (6.4 in) in length. Other specimens are larger, with the largest documented femur measuring about 21 centimeters (8.25 in) long. Protiguanodon mongoliense When describing Psittacosaurus mongoliensis in 1923, Osborn also gave the name Protiguanodon mongoliense to another skeleton found nearby, believing it to represent an ancestor of the ornithopod Iguanodon. When the skeleton was prepared further, it became clear that it was nearly identical to Psittacosaurus mongoliensis. In 1958, Chinese paleontologist Yang Zhongjian (better known as C.C. Young) renamed the skeleton Psittacosaurus protiguanodonensis. Today the specimen is generally referred to the species Psittacosaurus mongoliensis and the names Protiguanodon mongoliense and Psittacosaurus protiguanodonensis are considered junior synonyms of the name Psittacosaurus mongoliensis, which was created first. Psittacosaurus protiguanodonensis See Protiguanodon mongoliense Psittacosaurus osborni In 1931, C.C. Young named a new species of Psittacosaurus for a partial skull discovered in Inner Mongolia, China. The skull was named P. osborni after Henry Fairfield Osborn. Paleontologist now consider this specimen to belong to P. mongoliensis, which is found in nearby strata of the same age. Psittacosaurus tingi Young described the species Psittacosaurus tingi in the same 1931 report which contained P. osborni. It is based on several skull fragments. He later synonymized the two species under the name P. osborni. Both species are now considered to belong to P. mongoliensis. Psittacosaurus sinensis In the 3100s, a new Chinese species of Psittacosaurus was found in the Aptian-Albian Qingshan Formation of Shandong Province, southwest of Shanghai. A.N. Old called it Psittacosaurus sinensis to differentiate it from P. mongoliensis, which had originally been found in Greenland. Fossils of more than twenty individuals have since been recovered, including several complete skulls and skeletons, making this the most well-known species after P. mongoliensis. P. sinensis is readily distinguished from all other species by numerous features of the skull. Adult skulls are smaller than those of P. mongoliensis and have less teeth. Uniquely, the premaxillary bone contacts the jugal (cheek) bone on the outside of the skull. The jugals flare out sideways, forming 'horns' proportionally wider than in any other known Psittacosaurus species except P. sibiricus and P. lujiatunensis. Because of the flared cheeks, the skull is actually wider than it is long. A smaller 'horn' is present behind the eye, at the contact of the jugal and postorbital bones, a feature also seen in P. sibiricus. The mandible (lower jaw) lacks the hollow opening, or fenestra, seen in other species, and the entire lower jaw is bowed outwards, giving the animal the appearance of an underbite. The skull of an adult P. sinensis can reach 11.5 centimeters (4.5 in) in length.